We have produced an interventional cardiac catheterization system (SBDX) which provides 70% of the signal-to-noise ratio (SNR) of coronary angiography with only 15% of the patient skin dose, relative to conventional systems. We propose to modify this system to provide 100% of the SNR of a conventional system at 15% of the patient skin dose. This fluoroscopic system also has a unique ability to locate vessels and interventional devices in 3D at 30 frames/sec. We propose to use this ability to enhance the performance of two important clinical procedures: accurate balloon and stent sizing during coronary angioplasty and treatment of left atrial fibrillation using radio-frequency (RF) catheter ablation. AIM 1 - The increase in SNR will be accomplished by the construction of a new detector of the same design, but 2x greater area. We will demonstrate that we have reached our goal by comparison of image quality metrics and the results of porcine imaging studies on both SBDX and a state of the art conventional system. AIM 2 - We will demonstrate the ability of SBDX to perform accurate vessel and device sizing during the catheterization procedure, so that it may aid the interventionalist in the delivery of therapy. AIM 3 - We will demonstrate the ability of SBDX to aid the electrophysiologist with the localization of cardiac structures by providing real-time 3D catheter tracking and accurate registration of CT and fluoroscopic images during the procedure. Heart disease is a significant cause of morbidity and mortality in our society. Over 500,000 coronary angioplasties and 100,000 RF catheter ablation procedures are performed annually in our country, saving many lives and improving the quality of many more. The successful outcome of this research will provide therapeutic tools which improve the outcomes of these procedures, while dramatically lowering the radiation exposure to the patient and medical staff.